Chemical Reactions: Fast and Slow Investigating how surface area affects the rate of a reaction
Rhubarb contains a number of acids that give it a sour taste – this is why sugar is added to the vegetable when making a rhubarb tart. One of the acids found in the stalks of rhubarb is called oxalic acid. Oxalic acid reacts with a dilute solution of potassium permanganate and causes the pale pink of the potassium permanganate to become colourless. Oxalic acid is also found in Brussels sprouts, carrots and parsley.
1. Using this information, plan, design, carry out and write up an investigation to find out how increasing the surface area of rhubarb affects the rate at which potassium permanganate loses its colour. While designing this investigation, consider the following:
a. What variables will you keep the same? b. What variable will you change? c. What safety precautions do you need to take? d. What will your results table look like? e. What type of graph will you draw?
12.4
z
Fig. 12.4.6 Rhubarb
2. How could this investigation be changed to show that concentration affects the rate of a reaction?
Catalysts and Rate of Reaction A catalyst is a substance that speeds up the rate of a chemical reaction.
At the end of the reaction, a catalyst is chemically unchanged, meaning it can be used over and over again.
A catalyst works by lowering the amount of energy needed for a reaction to happen (activation energy). Catalysts make it easier for particles to react, as they will need less energy to get over the barrier.
The energy profile diagram in Fig. 12.4.8 shows how a catalyst lowers the activation energy needed for a reaction. Can you tell if it is an endothermic or an exothermic reaction? Explain your answer.
Energy
Activation energy without catalyst
Reactants
Activation energy with catalyst
z Fig. 12.4.7
Catalysts make it easier for particles to react
Learn more about catalysts in Biological World 3.1
z Products Time Fig. 12.4.8
Energy profile diagram showing how a catalyst lowers the activation energy of a reaction
